1. Field of the Invention
The present invention relates to an imaging apparatus, such as a digital camera or a video camera.
2. Description of the Related Art
A phase difference detection based auto focus (AF) (hereinafter, referred to as “phase difference AF”) system is employable for an imaging apparatus (e.g., a camera). The phase difference AF imaging apparatus halves the light flux having passed through an exit pupil of a photographic lens and causes a pair of focus detection sensors to receive halved light fluxes. Then, the imaging apparatus obtains a deviation of the photographic lens from an in-focus position (hereinafter, referred to as a defocus amount) by detecting a shift amount between signals output based on the light-receiving amounts, more specifically, by detecting a relative shift amount between the output signals in a light flux division direction (hereinafter, referred to as “image shift amount”).
As discussed in Japanese Patent Application Laid-Open No. 2001-305415, it is conventionally known to configure an image sensor to have a phase difference detection function to realize high-speed phase difference AF processing without using a dedicated focus detection sensor. According to the configuration discussed in Japanese Patent Application Laid-Open No. 2001-305415, the image sensor includes a pixel whose photoelectric conversion portion is substantially halved to realize a pupil-dividing function. The image sensor performs focus detection processing by individually processing the outputs to be obtained from the halved photoelectric conversion portions and uses a summed-up output of the halved photoelectric conversion portions as an image signal.
Further, as discussed in Japanese Patent Application Laid-Open No. 2010-271419, it is conventionally known to employ a configuration capable of reducing the influence of noise when a photographic subject is dark in a focus detection using the photoelectric conversion portion having the pupil-dividing function. The configuration discussed in Japanese Patent Application Laid-Open No. 2010-271419 includes detecting the brightness for each line of an image sensor and adding a line if the detected brightness is lower than a predetermined value.
However, according to the configuration discussed in Japanese Patent Application Laid-Open No. 2001-305415, the image sensor is required to realize a focus detection function and an imaging function simultaneously. Therefore, if only the shooting conditions are taken into consideration in determining an exposure setting value to control the image sensor, the setting value may be inappropriate for the focus detection. Further, in a case where an operator arbitrarily switches or changes the exposure setting value, the focus detection may not be successfully completed if the setting value is inappropriate. According to the configuration discussed in Japanese Patent Application Laid-Open No. 2010-271419, it is feasible to reduce the amount of noise included in the image signal to be used for the focus detection by performing the line addition processing based on the image signal. However, the adverse effect of the line addition that reduces the resolution of the photographic subject in the vertical direction is not taken into consideration.